Note: As of July 9, 2018, Blackbaud's School Management solutions (which include Core, onBoard, onCampus, onMessage, and onRecord) are utilizing a new top level navigation. This primarily affects users performing administrative tasks-constituent navigation for most parents, students, faculty, and alumni will remain the same. We appreciate your patience as we work to update our documentation. If you are seeing this message, the content of this solution is still being vetted for possible updates. Please refer to this mapping document if you need assistance finding a task in the new navigation.
There are times when sending that a particular ISP will be busy or be having issues - and our servers (at BBCS) will attempt for a certain period of time to deliver the message - and in the event that they cannot deliver in the time limit, it will abort the process and messages like this will pop up and it just means that the email server (ex. Verizon, AOL, Gmail, etc) wasn't accepting the messages at the time.
You'll want to try to resend to the addresses, and it's likely they'll send without issue.